The treatment of wood by impregnating it with preservatives and other agents, such as for fire-proofing, is very well known. One of the problems of the impregnation treatment is ensuring that the agent has impregnated the wood fully or at least sufficiently.
It is also known to prepare wood for impregnation by using steam to raise the temperature and pressure in the wood and break down some of the wood cells (after the pressure is rapidly reduced) and thereby to improve the natural permeability of the wood. Updated steaming specifications for, for example, round wood radiata pine steam conditioning recommend the use of steam at a temperature of 127° C. and pressure of 138 kPa for a period of from 1.5 to 18 hours depending upon the thickness of the wood before the pressure is rapidly reduced. While this treatment does help to improve the impregnation process, it has several disadvantages, particularly the duration of the treatment and the requirement for high pressure steam.
It is also known to dry wood and to form fibres by destroying the wood structure using microwave energy. It is important in drying the wood using microwave energy not to damage the wood and special drying schedules have been proposed for different wood species. In all of these schedules the microwave radiation intensity is very low, below 5 to 10 W/cm2, in order to avoid damage to the wood.
Destruction of wood using microwave energy to form fibres is performed at a considerably higher radiation intensity, for example up to 150 kW/cm2, with the aim of heating the moisture in the wood to form steam very quickly and in sufficient quantity to entirely break down the structure of the wood.
International Patent Publication No. WO99/64213 discloses a method which comprises subjecting wood to microwave radiation to cause water in the wood to vaporise resulting in an internal pressure in the wood such that the permeability of the wood is increased by partial or complete destruction of ray cell tissue, softening and replacement of wood resin, formation of pathways in the radial direction of the wood and/or by creating, on the base of destroyed rays, cavities in the wood, said cavities being primarily in radial-longitudinal planes of the wood. This reference also suggests that the microwave modified timber can have regions of different permeability which can alternate in radial, tangential and longitudinal directions.
The use of this method for pre-drying treatment may lead to 6–25% reduction of strength properties of the timber. As such, in some cases it is not adoptable. For example, the working side of parquet boards must have high hardness. After the MW treatment considered by the above prior art, that surface may lose its hardness to an extent which would make it unsuitable for use as an exposed surface.
During the MW treatment of large cross section sawn timber the core is heated faster than the shell (if supplying MW energy to the timber from all sides). As such, the vapour pressure may be high in the core, but the surface layers still may have no pathways in the radial directions for releasing vapours created in the core. This situation leads to the appearance of big checks in shell layers of the timber which spoil the material.
The growth and shrinkage stresses which take place in the wood may lead to losses of timber and may also lower the quality of the material. For example, during wood drying the shrinkage in the tangential direction is much higher compared with the shrinkage in the radial direction. Therefore, back-sawn lumber may experience cupping (transverse warping) after drying.
These shortcomings are advantageously alleviated using the method of microwave treatment according to the invention.